1. Field of the Invention
This invention relates to improvements in the catalytic hydrocracking of heavy hydrocarbon feeds to produce lower boiling products. In one aspect, this invention relates to removal of foulant from hydrocracking process streams. In still another aspect, this invention relates to a method of reducing fouling of hydrocracking process equipment.
2. Description of the Related Art
Hydrocracking converts components of a heavy hydrocarbon feed to lighter, more valuable products by contacting the feed with hydrogen, in the presence of a hydrocracking catalyst, at an elevated temperature and pressure. The hydrocracker effluent is cooled and depressurized, and the products of the conversion are separated from unconverted compounds.
The unconverted portion of the reactor effluent is recycled through the reactor to seek complete conversion. The unconverted-recycled portion contains convertible hydrocarbons which are converted during a subsequent pass through the reactor; however, it also contains hydrocarbons, which are substantially or wholly nonconvertible, which are resistant to cracking at hydrocracking conditions and hydrocarbons which are not cracked during recycling since they will not crack under hydrocracking conditions. As a result of the recycle, these cracking-resistant and noncracking compounds increase in concentration in the reactor effluent and become foulants. The foulants cause problems, as they accumulate in the process and precipitate during cooling of the reactor effluent since the foulants have a relatively low solubility in the reactor effluent. The precipitated foulants deposit in the cooling equipment and in cooler process piping. As a result, the hydrocracker runs are shortened, adversely impacting equipment utilization and process economics.
Although hydrocrackers can be designed to operate with a variety of feeds, catalysts, equipment configurations, and other design parameters, the fouling problems associated with hydrocrackers employing recycle streams is well-known to those skilled in the art.
Prior art methods addressed to the problems associated with hydrocracker fouling, have sought to reduce the concentration of polycyclic compounds, principally heavy polynuclear aromatic compounds ("HPNAs"), in hydrocracker streams. In prior art processes, it has been conventional to minimize the concentration HPNAs by hydrogenation or other conversion of those compounds.
U.S. Pat. No. 4,921,595 to Gruia claimed reduction of the concentration of 11+ ring HPNAs in a hydrocracking effluent by converting the compounds by hydrogenation using a zeolitic hydrogenation catalyst having pore openings in the range of about 8 to about 15 Angstroms and a hydrogenation component operated at the specified conditions to reduce the concentration of the HPNAs.
U.S. Pat. No. 4,931,165 to Kalnes claims a method of reducing hydrocracker process unit fouling by flashing a slipstream of recycled hydrocarbon liquid containing HPNAs with a hydrogen-rich gaseous stream to generate a concentrated liquid stream of HPNAs and a vaporized stream containing hydrogen and having a reduced concentration of HPNAs. The resulting vaporized streams are hydrogenated in a hydrogenation zone to convert and thereby further lower the concentration of the HPNAs.
U.S. Pat. No. 3,691,063 to Kirk teaches removal of asphaltic material from hydrocracker feed by use of a guard case containing high surface area catalysts such as alumina and various acid cracking catalysts such as silica alumina and various modified silica aluminas including zeolites. The guard case is operated at a temperature of 600.degree. F. to 1,000.degree. F. and a pressure in the range of about 10 to 50 psig.
U.S. Pat. No. 4,618,412 to Hudson, et al., removes polynuclear aromatic hydrocarbon compounds to suppress fouling of the hydrocracker unit by contacting such compounds with an iron catalyst in the presence of hydrogen to hydrogenate and hydrocrack and convert the material to be removed.
U.S. Pat. No. 4,447,315 to Lamb, et al., claims reducing the concentration of polynuclear aromatic compounds by contacting the unconverted hydrocarbon stream with an adsorbent which selectively retains polynuclear aromatic compounds. Lamb et al. discloses that the adsorbent can be silica gel, activated carbon, activated alumina, clay and the like.
U.S. Pat. No. 4,411,768 to Unger, et al., is relevant for its teaching, in a hydrogenation process, the treating of liquid recycle streams to remove coke precursors by cooling the liquid recycle in to a temperature of from 350.degree. F. to 600.degree. F., with such cooling separating coke precursors from the liquid recycle. Coke precursors, which are characterized by Unger as being toluene insolubles and heptane insolubles, precipitate from the liquid recycle at such cool temperatures. Removal of the coke precursors was by filtration, centrifugation or by adding a low-boiling liquid to the liquid recycle to reduce the solubility of the coke precursors.
U.S. Pat. No. 3,619,407 to Hendricks, et al., reduces polycyclic aromatic hydrocarbons, characterized as benzocoronenes, from hydrocracking effluent by partially cooling the effluent to condense a portion of the normally liquid hydrocarbons to form a benzocoronene-rich partial condensate and withdrawing a bleedstream of the benzocoronene-rich material from the reactor effluent.
U.S. Pat. No. 4,775,460 to Reno teaches removal of polycyclic aromatics by a two-step procedure. The first step of the procedure comprises contacting the hydrocarbon feedstream with a material which promotes the formation of the polycyclic aromatic hydrocarbons at conditions of elevated temperature but which are mild (low pressure) relative to hydrocracking conditions, and the second step of the procedure is removal of the polycyclic hydrocarbons by an absorbent material such as activated charcoal.
U.S. Pat. No. 4,902,405 to MacLean, et al., removes materials, from a hydrocracking zone product stream, having a boiling range from about 500.degree. F. to about 650.degree. F., which MacLean et al believed to contain precursors for heavy materials boiling over 1050.degree. F., and materials having an initial boiling point above 1050.degree. F.
Other methods to reduce hydrocracker system fouling are needed.